For example, brain astrocyte-derived exosomes can promote the outgrowth of brain metastatic cancer cells by transferring PTEN-targeting miR-19a to these cancer cells.26 Lyden et al.27 demonstrated that the exosomes derived from organotropic metastatic cancer cells can be preferentially up-taken by specific host organ cells to prepare the PMN. The gene discussed is PTEN; the disease is cancer.